ANOMALOUS TEMPERATURE BEHAVIOR OF RAMAN-SPECTRA FROM VISIBLE-LIGHT EMITTING POROUS SILICON

We performed a Raman scattering study on porous Si which exhibited red light emission. Porous Si membranes (100-200 mum) were formed by anodic dissolution (current density = 70-100 mA/cm2) of (111) oriented, heavy boron doped, p-type Si wafers. The membranes display red light when illuminated with white or flash light. They possess a major Raman line with the wavenumber position near, but slightly lower than that of crystalline (c-) Si. This line width is broader than that of c-Si, and narrower than, or comparable with, that of micro-crystalline (muc-) Si. Its temperature behavior is quite different from that of c- and muc-Si. The Raman line from our porous Si shifts down in wavenumber and broadens with a decrease of temperature from 300 to 80 K, which is characteristic of the soft-mode nature, and is opposite to that of c-Si and muc-Si. Three possible mechanisms responsible for this anomalous temperature behavior of the Raman spectra from porous Si are discussed qualitatively.